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West Nile Virus Physician Instructions

Physician Instructions for Submission of Specimens to the Oklahoma State Public Health Laboratory for West Nile Virus Testing

Clinical presentation
Most people who become infected with West Nile virus (WNV) develop no clinical illness or symptoms (approximately 80%).  Of the approximately 20% of infected people who do develop symptoms, most will experience “West Nile Fever.” Usually lasting 2-10 days, West Nile Fever is a vague syndrome with typical symptoms of fever, headache, marked lethargy, and myalgias possibly accompanied by a maculopapular rash on the trunk of the body. Since this illness is self-limiting, testing for WNV in this clinical setting is up to the physician’s discretion. Older adults, especially those over 50 years of age, are at greatest risk of having the illness progress to encephalitis or other manifestations of neuroinvasive disease.

Laboratory diagnosis
The most sensitive screening test for WNV infection is the IgM-capture ELISA performed on both cerebrospinal fluid (CSF) and serum. The IgM-capture ELISA test of CSF in patients with WNV encephalitis should be positive shortly after the onset of symptoms until the eighth day of illness. The IgM capture test on serum is less sensitive in the very acute phase of infection, so patients with a negative acute serum and compatible clinical presentation should also have the test performed on a convalescent serum specimen collected 2-3 weeks after the first collection date. Because serological cross-reactivity is often observed in flavivirus infections (particularly between WNV and St. Louis encephalitis virus), any samples that demonstrate indistinguishable antibody titers to more than one flavivirus will be tested by plaque reduction neutralization testing for confirmation of the infecting virus.

The Oklahoma State Department of Health (OSDH) Public Health Laboratory (PHL) also has the capability for diagnosis of WNV infection by polymerase chain reaction (PCR) testing; however, this diagnostic test is only applied to CSF or tissue. Physicians who submit a CSF specimen should also submit a serum specimen (for IgM ELISA) from the case patient due to the lower sensitivity of the PCR test.

Reporting and Specimen Submission for Suspect Cases
WNV testing is currently available free of charge at the OSDH PHL. Oklahoma physicians are asked to submit specimens for WNV testing on hospitalized patients with potential mosquito exposure and any of the following conditions:

  • Any adult or pediatric patient displaying:  a) Fever > 38.0°C or 100°F; b) CNS abnormalities (altered mental status, photophobia, palsies, or paralysis); c) CSF findings of pleocytosis with predominant lymphocytes, negative gram stain, and/or elevated protein.
  • Any adult or pediatric patient with a presumed diagnosis of acute viral encephalitis or with focal CNS findings and a fever.
  • Any adult or pediatric patient with fever and presumed Guillain-Barre' Syndrome or acute flaccid paralysis.
  • Any adult patient admitted with presumptive aseptic meningitis (fever, headache, stiff neck, and/or other meningeal signs; CSF pleocytosis with predominant lymphocytes and moderately elevated protein; a negative gram stain; and a negative culture to date).

For information pertaining to specimen requirements and handling instructions, contact the Immunology Section, OSDH PHL at (405) 271-5070. To report suspect cases or obtain case consultation, contact the Acute Disease Service epidemiologist-on-call at (405) 271-4060.


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